In the field of optical transmission technology, the wavelength division multiplexing (WDM) transmitting a plurality of optical signals of different wavelengths has been put into practical use in recent years, and the technology is being improved still more.
FIG. 1 shows a configuration example of wavelength division multiplex (WDM) transmission equipment in a WDM optical transmission system, in which two sets of neighboring relay equipment are shown among a plurality sets of relay equipment connected in tandem.
In neighboring stations A, B each constituted of such WDM transmission equipment, signal light output from the station A is transmitted to the station B on a transmission line 202. The signal light is input into a receiving amplifier (pre-amplifier) 111 in a receiving amplifier unit 120 of the station B.
The signal light amplified in receiving amplifier 111 is then demultiplexed by a wavelength demultiplexer (DMUX) 112 into signal light of different wavelengths, and the signal path is selected (so as to pass through or add/drop) in an optical switch 113.
As for wavelength light having passed through optical switch 113, the level is adjusted for each wavelength in a variable optical attenuator (VOA) 114, and input into a wavelength multiplexer (MUX) 115 provided in a transmitting amplifier unit 130. The light is wavelength-multiplexed in wavelength multiplexer (MUX) 115, amplified in a transmitting amplifier (post-amplifier) 116 of a transmitting amplifier unit 130, and further transmitted to a non-illustrated succeeding station located on the east side through a transmission line 203.
Here, receiving amplifier (pre-amplifier) 111, as well as transmitting amplifier (post-amplifier) 116, is provided with an optical amplifier which uses excited light produced by a laser diode (LD). The amplification factor is controlled by the amount of laser diode (LD) current.
Meanwhile, wavelength light dropped in optical switch 113 is transmitted to another network through a transmission line 207. Also, the level of a newly added wavelength light input from a transmission line 206 and added in optical switch 113 is adjusted in variable optical attenuator (VOA) 114 and then wavelength-multiplexed in wavelength multiplexer (MUX) 115, in a similar way to the aforementioned passing-through wavelength light. The light is also amplified in transmitting amplifier 116 and further transmitted to the non-illustrated succeeding station located on the east side through transmission line 203.
Similarly, signal light output from the station B to the station A is transmitted through a transmission line 212, and input into a receiving amplifier 141 provided in a receiving amplifier unit 230 in the station A. The signal light amplified in receiving amplifier 141 of the station A is demultiplexed to each wavelength in a wavelength demultiplexer (DMUX) 142, and the signal path is selected (so as to pass through or add/drop) in an optical switch 143. The passing-through signal light, as well as signal light transmitted from a transmission line 216 and added in optical switch 143, is level-adjusted for each wavelength in a variable optical attenuator (VOA) 144, wavelength-multiplexed in a wavelength multiplexer (MUX) 145 of a transmitting amplifier unit 240, amplified in a transmitting amplifier 146, and transmitted to a non-illustrated succeeding station on the west side through a transmission line 213.
In such a way, by way of example in the conventional art, WDM transmission equipment is connected in tandem, through which optical transmission is performed bi-directionally, as well as add/drop of optical signals (for example, refer to Japanese Patent Number 3,241,337).
Here, in both receiving amplifier and transmitting amplifier provided in WDM transmission equipment, it is required to amplify optical signals so that a signal level becomes constant for each wavelength. For this purpose, it is necessary to achieve an appropriate gain (degree of amplification) setting in each amplifier.
By way of example, in transmitting amplifier 106 provided in transmitting amplifier unit 110 of the station A, and also in receiving amplifier 111 provided in receiving amplifier unit 120 of the station B, the gain can uniquely be determined because the optical signal is input into transmitting amplifier 106 after each level of the wavelength light is adjusted in variable optical attenuator (VOA) 104.
However, as for receiving amplifier 111 in the station B, an input light level depends on a transmission line loss, etc. produced in transmission line 202. Therefore, when the power of receiving amplifier 111 is turned on, and when a fiber is replaced or a break of the fiber is restored in transmission line 202, it is necessary to determine the gain of receiving amplifier 111 so as to fit the input level into receiving amplifier 111.
Here, in order to set the gain of receiving amplifier 111 correctly, it is necessary to input light having a stable level with a constant number of wavelengths into receiving amplifier 111 while the gain setting of receiving amplifier 111 is in progress.
For this purpose, it is required to stabilize the light output from transmitting amplifier 106 and to supply stable light with a constant number of wavelengths to receiving amplifier 111, by supplying stable input light with a constant number of wavelengths to transmitting amplifier 106.
To cope with the above-mentioned requirement, in case of providing input light having a constant number of wavelengths with stable light level to transmitting amplifier 106, there may be a method of stopping passing-through light transmitted from the preceding station to the station A, and setting added light by means of optical switch 103 in the station A, or, alternatively, a method of preparing a light source 107 to produce reference light and thereby inputting constant light to transmitting amplifier 106.
As a method for stopping the passing-through light from the preceding station to the station A, there may be a method of dropping the passing-through light entirely to transmission line 205. Or, alternatively, it may be possible to attenuate the passing-through light invariable optical attenuator (VOA) 104.
However, according to the methods described above, it is necessary to prepare a light source to be connected to transmission line 204 for added light, or light source 107 for supplying the reference light. It causes a problem of increased cost for preparing such light sources throughout the stations connected in tandem.
Also, in the case of setting the receiving amplifier gain by use of a light source for starting up the receiving amplifier, considering a receiving amplifier in a span with no light source prepared, there is a method of setting the gain using output light of the transmitting amplifier located in the preceding span. In this method, the gain setting of the receiving amplifier concerned will be performed after the gain setting for the receiving amplifier in the preceding span is completed.
However, according to this method, the gain settings must be performed successively from the receiving amplifier in the span in which the light source is provided. Therefore, this method causes another problem of taking substantial time for the entire spans to complete the gain settings for the entire receiving amplifiers.
Also, when setting the receiving amplifier gain in a span having no light source, if other spans are in operation for service, there is a problem of requiring temporary suspension of the service ranging from a span having the light source to a span having no light source, in order to perform the gain setting of the receiving amplifier which has no light source.
Further, when stopping the passing-through light to transmitting amplifier 106 by use of variable optical attenuator (VOA) 104, this variable optical attenuator (VOA) 104 cannot completely attenuate the passing-through light, and produces leak light. When the leak light is produced, a problem of an unstable input to receiving amplifier 111 arises, which impedes correct gain setting. Accordingly, it becomes an issue to shut off completely the leak light which leaks to transmitting amplifier 106.
Moreover, when the gain setting of receiving amplifier 111 is required, the necessity of the gain setting of receiving amplifier 111 has to be recognized by a maintenance person, and the input light for setting the gain of receiving amplifier 111 has to be set. After the gain setting procedure for receiving amplifier 111 is completed, it is also necessary for the maintenance person to restore the input light having been used for the gain setting of receiving amplifier 111, and instruct signal light setting. As such, the maintenance person has to intervene frequently to complete the receiving amplifier gain setting. This produces a load to the maintenance person, as well as an increased possibility of an operational error.
Also, when setting the gain of receiving amplifier 111, there arises a problem that signal light of a wavelength identical to the wavelength of the light for gain setting is corrupted (namely, the signal light becomes useless). To cope with this problem, when setting the gain of receiving amplifier 111, it becomes necessary to input the input light having a stable light level to receiving amplifier 111 without changing the setting of the existent signal light.